Liquid metal corrosion (LMC) has been identified as one of the main problems in relation to molten-metal cooled nuclear systems, such as lead fast reactors (LFRs), accelerator driven systems (ADSs) or fusion reactors. Such corrosion is a physical or physicalchemical process involving, inter alia, the dissolution of steel constituents and their transport in the liquid and solid phases. LMC may alter the microstructure, composition, and morphology of steel components, with an adverse effect on the mechanical performance thereof.
One of the main approaches to mitigate the corrosion effects is to promote the in situ formation of iron and chromium oxide protective layers on the surface of the steels, typically austenitic or ferritic/martensitic steels. Such a solution can be achieved by injecting oxygen into the coolant and by an accurate control of the concentration thereof in the liquid metal. However, such a method is not reliable for temperatures exceeding 500° C.
In the last decades, research focused on high temperature corrosion mechanisms and methods for structural material protection in fourth-generation plants. All over the world, experimental studies have been carried out in these fields, mainly targeting oxygen control techniques and the development of alumina coatings, aluminization surface treatments, or even new materials, such as oxide dispersion-strengthened steels (ODSs).
In spite of such efforts, the protection of structural steels against HLM corrosion still remains an unresolved issue.
In this regard, the use of ceramic coatings could be a valid option. However, while an efficient use thereof as an environmental barrier would require chemical stability, compactness, stiffness, high wear resistance, excellent adhesion and a strict correspondence with the mechanical properties of the steels, meeting all these requirements by the conventional industrial techniques is difficult, especially at low operating temperatures.
In particular, the present invention relates to a nuclear fuel cladding tube for a liquid-metal or molten-salt cooled reactor, said cladding tube comprising a tubular body in metal material and a protective coating applied on an outer surface of the tubular body, intended, in use, to contact the coolant.
It is known that the nuclear fuel cladding is one of the components intended to be subjected to the most severe conditions, since it is provided to operate, in the advanced nuclear systems, at temperatures that could reach 800° C.
In such regard, from WO 2007/000261 a technique is known, providing for the development of thin protective alumina coatings by an aluminization surface treatment and the subsequent selective oxidation of steel, dictating a suitable oxygen concentration in the liquid metal. However, Al diffusion in steel, an insufficient Al content in the surface alloy, due to electronic pulse overlap during the GESA treatment (which evaporate a considerable amount of that metal), and the “fretting corrosion” phenomenon, which occurs at the contact point between the fuel rods and the spacer grids, following the repeated contact between rod and grid (which disrupts the very thin alumina layer by its mechanics, which is not known or at least not fully identified), can lead to a failure of the surface protective layer, thus to the steel exposure to the liquid metal, resulting in corrosion.
Therefore, an object of the invention is to propose a protection for the nuclear fuel cladding that allows overcoming the above-mentioned drawbacks.